This invention relates generally to techniques in joining together metal sheets and to adhesives. More particularly, the invention relates to a new and improved adhesive for forming a strong lap joint in a very short time in the process of joining the surfaces of metal sheet to metal sheet by means of an organic adhesive.
The term "metal sheet" as used herein is intended to cover a wide variety of metal sheets, important examples of which are iron and steel sheets, aluminum sheets, sheets of other metals, these metal sheets plated over their surfaces with metals such as zinc, tin, chromium, and aluminium, metal sheets chemically or electrolytically treated with chromic acid or phosphoric acid and metal sheets coated over their surface with an organic paint.
Metal sheets of the above stated character can be securely joined by the metal-sheet adhesive of the invention, which adhesive is prepared by physically mixing a crystalline, linear homopolyamide and a semicrystalline copolyamide and is used directly or as a solution thereof in a suitable solvent, as described more fully hereinafter.
Linear polyamides are known for their high physical strength and extremely tough nature in comparison with other plastics, and are therefore used singly or in combinations with other substance as adhesives for bonding metals and other substances. Examples of these combinations are admixtures of linear polyamides with novolak type phenolic resins and the like (for example, as disclosed in Japanese Patent Publication No. SHO 36-20569), thermosetting type adhesives prepared by adding epoxy resins and other hardening accelerators to linear polyamides, adhesives comprising linear polyamides coated with epoxy resins or diisocyanates to be thermoset at the time of bonding (for example, as disclosed in Japanese Patent Publication Nos. SHO 35-4221 and SHO 36-15636), and copolymers produced by grafting to linear polyamides other components (for example, as disclosed in Japanese Patent Publication No. SHO 40-17662).
Adhesives produced by admixing novolak type phenolics with linear polyamides, however, are deficient in high-speed or quick adhesion property with respect to metal sheets. This deficiency is all the more pronounced in adhesives produced by changing the character of a polyamide and rendering it into a thermosetting resin, such resins requiring a long time for setting and therefore being unsuitable for quick-bonding uses. Furthermore, copolymers formed by grafting other components to linear polyamides entail high production costs and are therefore not practical at present.
In order to complete quick-bonding procedures in rapid manner, it is obviously necessary that the adhesive has, as one of its physical properties, good adhesion with respect to the articles to be bonded. As a result of our research on the solidification phenomenon occurring when adhesives are cooled from the molten state, we have discovered that, in addition to this good adhesiveness, a high temperature dependency of the melt viscosity of the adhesive and the fact that the adhesive is one of a crystalline high polymer are important conditions for quick-bonding procedures.